1. Field of the Invention
The present invention relates to apparatus for the adsorptive fractionation of a gaseous mixture, and more particularly to a twin tower dryer apparatus finding particular but not necessarily exclusive use for drying compressed air.
More specifically, the present invention relates to an improved method of controlling the adsorption and desorption of the sorbent beds of a twin tower fractionation apparatus by controlling substantially only the open and closed condition of two purge valves operatively associated with the twin tower fractionation apparatus.
2. Brief Description of the Prior Art
Twin tower gas adsorbers have been marketed for many years and are in widespread use for a variety of processes involving the adsorptive separation of gaseous components, such as for the removal of moisture from compressed air. Apparatus of this type generally comprises two adsorbent or desiccant beds, one of which is adapted to be on an adsorption or drying cycle, while the other is being purged of the adsorbed component and regenerated for reuse. In the drying of compressed air, for example, air to be dried is passed through one bed and, when the adsorptive capacity of that bed is reached or approached, the air is cycled to the other bed while the first bed is purged of moisture, usually by utilizing a lower pressure purge gas flow tapped from the dry air outlet.
Many forms of control devices have been utilized for such twin tower dryers. Basically, appropriate valves are utilized to direct the air or gas flow through the dryer in a predetermined cycle, with control or timer devices being provided to produce the desired cycles. Control valves are utilized which are actuated in any suitable manner such as mechanically, electrically by solenoids, electronically, pneumatically, or otherwise. A variety of timers can be employed, all of which are generally equivalent and produce the desired timed cycles. Among the types of timers are mechanical clocks, electrical or electronic timers, and microprocessors.
The advent of microprocessors has led logically to the use of such data processing devices for control purposes. Such devices are able to handle more complex data and hence it is possible to incorporate into the timing cycle the consideration of various factors such as moisture content, pressures and temperatures, and to adjust the cycle according to variations in such parameters. Those of ordinary skill in the microprocessor art are readily able to utilize such technology for the control of a wide variety of ohemical and physical processes, including the adsorption of gases and regeneration of adsorbent beds. Timing devices including mechanical, electrical, electronic, pneumatic or microprocessor timing devices are equivalent and may be utilized effectively. Likewise, various types of control valves, such as solenoid actuated valves, pneumatic valves, pilot actuated valves and the like, may be utilized to advantage.
Twin or multiple tower gas adsorption apparatus may be utilized for many different chemical processes and their use is not limited to the drying of gases such as air, but extends generally to the fractionation of various gaseous mixtures. Gas adsorption is an ancient process which has been utilized in many chemical processes. Purging or regeneration of the adsorbent bed has been carried out in a variety of ways, including the use of heat as well as heatless purge cycles. Principally, the saturated sorbent bed is purged or regenerated by utilizing a gas stream low in the adsorbed material, and the purge is ordinarily carried out at a pressure lower than the pressure of the adsorption stage, and often at a temperature higher than that of the adsorption stage. In some instances, heat has been added to the purge stage to enhance the regeneration.
Twin tower dryer or gas adsorbing apparatus in which the towers are adapted for alternate adsorbing and purge operations are shown in the prior art. See, for example, U.S. Pat. No. 2,944,627, issued July 12 1960, to C. W. Skarstrom, for "Method and Apparatus for Fractionating Gaseous Mixtures by Adsorption"; U.S. Pat. No. 3,138,439, issued June 23, 1964, to C. W. Skarstrom, for "Apparatus and Process for Heatless Fractionation of Gaseous Constituents"; U.S. Pat. No. 3,258,899, issued July 5, 1966, to T. E. Coffin, for "Apparatus for Fractionating Gaseous Mixtures"; U.S. Pat. No. 3,552,096, issued Jan. 5, 1971, to P. Dayson, for "Apparatus for Drying Air or Other Gas"; and U.S. Pat. No. 3,323,292, issued June 6, 1967, to C. D. Brown, for "Apparatus for Fractionating Gaseous Materials."